Change debugserver to clear PAC auth bits manually

debugserver is currently using kernel supplied macros,
arm_thread_state64_get_{pc,fp,sp,lr} which can crash on an authorization
failure when the inferior has crashed with an invalid pc value, for
instance.  debugserver needs to be resistant to crashing in this
scenario, and we're merely clearing the bits, so do it with a bit
mask operation instead.

Differential Revision: https://reviews.llvm.org/D136620
rdar://98073271
rdar://100663221